Hey I'm a rising senior applying for ETA as well! I've been doing tons of research and asking a lot of questions so although I'm not an expert (and obviously not a successful applicant yet) I think I may have some input that you may find helpful. Personally I think you should follow your "heart" (cliche as it sounds). I'm assuming this means Ukraine which you have a more personal connection with, and I think this will actually be helpful in writing both statements. You said " I think it would be amazing to spread American English and culture while also learning more about my own culture." in Ukraine, and from all the webinars and info sessions I've attended, every single time the speakers always say the biggest goals of the Fulbright program is exactly this: to both share American culture and learn the culture of the country you're living in. I'm sure you can write a strong statement of purpose for Ukraine too, if you're worried start now sending drafts to your school's Fulbright advisors (if they provide essay review service) or if you have friends who won ETA for other countries, or anyone in general.
From a quick look at the website, Ukraine and Uruguay both offer 2 ETAs (you can find the stats for number of applicants each year somewhere on the website) so from this it looks pretty even. As long as you're not a Ukrainian citizen I think you should definitely apply for Ukraine. Make sure you're sure you aren't a Ukrainian citizen though, because I did not know until a few years ago that I was automatically citizen of my parents' country because they are both citizens, but rules do differ for each country.
Also, I have a friend who was selected as an ETA (for a different country) with a background very similar to yours. Both his parents are immigrants from the country he applied to, and he speaks the language, he's never been there before, and is hoping to learn more about his parents' country through the ETA experience. His area of study was science, so your area of study focusing on Latin America shouldn't be an issue at all, and (from what I've heard) having any previous ESL can only be a bonus! I think your passion for cultural exchange with Ukraine will show through in your essays. It may not be easy at first to find the right words, but I'm sure you can figure it out, and getting feedback on drafts (or just on ideas of what to write about) will help a lot!